Various proposals about electrically conductive fibers have heretofore been made. For example, known is an electrically conductive fiber having electrical conductivity imparted by plating metal onto the surface of a fiber having no electrical conductivity. However, there is a problem with such an electrically conductive fiber having a metal plating layer on its surface that electrically conducting performance is degraded because the plating layer on the surface readily exfoliates during a knitting/weaving process or its following process or the plating layer is readily dissolved and removed during a dyeing treatment or a refining treatment of textile.
A metal fiber is known as another type of electrically conductive fiber. It, however, has problems that metal fiber is generally high in cost and poor in spinnability; it causes troubles during the knitting/weaving process or dye-finishing process; it readily breaks or exfoliates in washing on practical use; and it readily gathers rust.
In place of such known technologies using metal, an electrically conductive conjugate fiber is known which is obtained by adding electrically conductive carbon black fine particles to a thermoplastic polymer, causing a resulting material to exist in the form of an electrically conductive layer on the surface of or inside a fiber continuously along the longitudinal direction of the fiber, and conjugate spinning the resultant with another fiber-forming thermoplastic polymer. However, in order to obtain electrically conducting performance by the thermoplastic polymer to which electrically conductive carbon black fine particles have been added (hereinafter, referred to as an electrically conductive layer), it is necessary to add a large amount of electrically conductive carbon black fine particles to the polymer. Addition of a large amount of carbon black fine particles causes a problem in which the spinnability and the stretchability of the polymer deteriorate drastically. As a method of solving problems caused by stretching, a method including no stretching is conceivable. However, when stretching is not performed, the fiber itself has low strength and the carbon black fine particles in the electrically conductive layer fail to form the structure described below, and therefore no satisfactory electrically conducting performance will be obtained. Moreover, such a method has a drawback that if the stretching is performed by force, the electrically conductive layer will be broken in the fiber or, even if it is not broken, the structure of the electrically conductive carbon black fine particles will be broken. Further, the electrically conductive layer will be broken readily when a slight external force is applied to the electrically conductive fiber, and as a result the electrically conducting performance will be lost.
There is another problem that an electrically conductive layer containing a large amount of carbon black fine particles shows low adhesiveness to another polymer forming the fiber and, as a result, interfacial peeling will occur readily during a process for producing woven/knitted fabrics or during use as an electrically conductive product to change the electrically conductive layer to a sole fiber and an electrically conductive layer with a low strength-elongation will be broken easily (see, for example, JP-A 57-29611 and JP-A 58-132119).
The problems with the above-described conventional electrically conductive fibers include that the strength of fiber itself is low, that an electrically conductive layer is readily broken, that no satisfactory electrically conducting performance is obtained, that an electrically conductive layer readily exfoliates, and that the conventional electrically conductive fibers are inferior in acid resistance and durability.
The present inventors have already filed on Jan. 11, 2006 a patent application (Japanese Patent Application No. 2006-003567) as an electrically conductive fiber which solves these problems. The invention relates to a fiber containing a polyethylene terephthalate as a major component. This is an item in which the fiber surface is substantially entirely covered with a polyester-based electrically conductive layer composed of the fiber's surface layer incorporated with carbon black fine particles. Moreover, the proportion of the electrically conductive layer in the fiber is adjusted within a range of 15% by weight or more. By obtaining such an electrically conductive fiber by a special spinning method, it is possible to obtain a fiber excellent in strength and elongation degree and to inhibit an electrically conductive layer from breaking. Furthermore, by using a polyethylene terephthalate-based resin as a fiber-constituting resin, it is possible to obtain a sheath-core conjugate fiber excellent in acid resistance and durability.
However, the present inventors found that the prior application can achieve fiber performance and electrically conducting performance which are greatly improved than conventional fibers, but it is not insufficient for fields where higher performance is required. They researched in order to find an electrically conductive fiber which highly satisfies required performance, achieving the present invention. That is, the present invention restricts the cross sectional shape of a fiber to a specified shape in the aforementioned already-filed invention. This can achieve initial performance and its durability at a higher level and it provides an effect superior to that of the invention of the prior application also in applications where higher performance is required.